The present invention relates to a zoom lens system that is suitable for use with a compact camera and having a small degree of constraint on back focus.
The following three types of zoom lens system have heretofore been proposed for use with compact cameras. The first type is a telephoto zoom lens composed of two lens groups, the first group having a positive focal length and having a stop diaphragm and the second lens group having a negative focal length. (Examples of lenses of this first type are shown in Unexamined Published Japanese Patent Applications Nos. 128911/1981 and 201213/1982, together corresponding to U.S. Pat. No. 4,682,860, Japanese Patent Application Nos. 48009/1985 and 170816/1985, and Japanese Application No. 191216/1985, corresponding to U.S. Pat. No. 4,659,186. The second type, which is a modification of the first type, is composed of three lens groups. The first group has a positive focal length and it has a stop diaphragm. The second lens group also has a positive focal length, while the third lens group has a negative focal length. (An example of this second type is Unexamined Published Japanese Patent Application No. 184916/1983). The third is composed of four lens groups, the first group having a positive focal length, the second group having a negative focal length, the third group having a positive focal length and the fourth group having a negative focal length (An example of this third type is illustrated in Unexamined Published Japanese Patent Application No. 57814/1985 corresponding to U.S. Pat. No. 4,701,033).
However, these three types of conventional zoom lens systems, while adapted for use with compact cameras, have certain problems, as described below. The first type which is composed of two lens groups has a very simple lens configuration, but the first and second lens groups must be moved by large amounts for zooming. The second type is a modification of the first type in that the first lens group is divided into two groups, each having a positive focal length. In this lens too the amount by which each of the lens groups must be moved for zooming is no smaller than in the first type. The third type, which is composed of four lens groups, has the advantage that the individual lens groups need to be displaced by smaller amounts than in the first and second types. However, this type of lens system is difficult to manufacture for two reasons. First, the overall lens arrangement is complex. Secondly, the four lens groups, particularly the second and third lens groups, have such great powers that small manufacturing differences can cause substantial adverse effects on the performance of the final product.
In practical applications, the zoom lens system of all three types have one common problem in that, since they are intended for use with compact cameras, their design does not include any constraint on back focus. In an extreme case, the back focus may be as small as about 1 mm. The absence of any constraint on back focus is advantageous for the purpose of reducing the overall length of the system but, on the other hand, the lens diameter of the rear group is increased and the overall size of the camera, including the mechanisms used for zooming, auto-focusing and the like, will be by no means be smaller than when certain constrains on back focus exist. Another problem associated with the absence of any constraint on back focus is an increase in the possibility of internal reflections between the film surface and the surfaces of lenses in the rear group.
In order to solve these problems of the prior art zoom lens systems for use with compact cameras, the applicant of the present application previously accomplished the invention described in Japanese Patent Application No. 232459/1985. In the embodiments described in the specification of that application, the system is capable of attaining zoom ratios of the order of 1.6. However, there is still room for improvement, namely, in the satisfaction of the demand for a zoom lens having an even greater zoom ratio. With a view to satisfying this need, the applicant later proposed in Japanese Patent Application No. 268473/1985 a lens system capable of attaining a zoom ratio exceeding 1.7. However, it is desired to provide a zoom lens system which requires fewer lens elements than the lens system of this application.